1. Field of the Invention
The present invention relates to an ignition coil assembly for an internal combustion engine, and particularly to an ignition coil assembly suited for an electronic distribution type ignition in a multicylinder internal combustion engine.
2. Description of the Related Art
Recently it is brought into practice to execute the ignition of a multicylinder engine without providing a mechanical distributor which has been popularly used. In such an ignition system, an electronic distribution function is substituted for a conventional rotating districution mechanism. Therefore, such a system is called an electronic distribution system, for the convenience of the following explanation.
According to a typical one of the electronic distribution system, as disclosed in Japanese Patent Laid-open No. 58-44707, for example, an ignition coil assembly having two primary coils wound reversely to each other in the winding direction and a secondary coil magnetically coupled with the primary ones is employed for ignition in a four cylinder engine. Usually, these coils are integrated and molded by an appropriate synthetic resin as disclosed in Japanese Patent Laid-open No. 56-75962, for example.
The two primary coils are alternately supplied with a current through power transistors which are connected with the respective primary coils and alternately rendered conductive by gate signals from an ignition control unit. As a result, the current flowing through one primary coil becomes an intermittent current. Every time the current flowing through the primary coils is interrupted, the high voltage is induced across terminals of the secondary coil. Because both the primary coils have the different winding direction from each other, the polarity of the high voltage induced in the secondary coil upon interruption of the current flowing through one of the primary coils is opposite to that of the voltage induced upon interruption of the current in the other primary coil. Namely, when the current of one of the primary coils is cut off, the high voltage is induced so as to be positive at one of the terminals of the secondary coil, and when the current of the other primary coil is cut off, it is induced so as to be positive at the other terminal thereof.
To each terminal of the secondary coil are connected two diodes which are in the opposite direction. Usually, these diodes are also molded in one body together with the coil portion. A cathode of a first diode and an anode of a second diode are connected commonly to one of the terminals of the secondary coil and similarly a cathode of a third diode and an anode of a fourth diode to the other terminal thereof. The remaining electrodes of every diodes are led to respective spark plugs.
With the above mentioned arrangement, the high voltage produced in the ignition coil unit is distributed to the four cylinders. In this case, it is to be noted that a spark discharge occurs in two cylinders simultaneously every generation of the high voltage in the secondary coil. However, if the direction of the diodes and the relation of the spark plugs connected thereto are properly selected, the spark discharge taken place in either one of the two cylinders can be rendered inoperative.
In the electronic distribution system as described above, diodes have to provided on the high voltage side of the ignition coil unit. Usually, an ignition coil unit is required to produce the voltage of more than 30 kV in order to secure a good performance of engines. Diodes used in such high voltage are considerably expensive. On the other hand, the cost of the coil portion has been extremely reduced by adopting a wide variety of manufacturing techniques. Therefore, the use of such expensive high voltage diodes contributes to raising the total cost of the ignition coil unit to a great extent and brings the every effort in the cost reduction of the electronic distribution system to naught.
In order to realize the inexpensive electronic distribution system, it has been considered to employ two separate ignition coil units. Although, since the coil portion itself is not costly, an assembly of such ignition coil units can be realized economically, the size thereof becomes considerably large and the problem occurs in its installation at an engine. To the contrary, if plural coil units are compactly assembled to be placed closely for the purpose of avoiding such a problem in the installation, the magnetic interference happens among the coil units to invite the miss in the ignition.